1. Field of the Invention
This invention relates generally to enclosures for electronic equipment, and more particularly, to the sealing of these types of enclosures for preventing the emission and/or entry of electro magnetic and/or radio frequency interference.
2. Discussion
Functioning electronic devices are known to produce electromagnetic interference (EMI) and radio-frequency interference (RFI) emissions. These emissions radiate outwardly from the electronic device and may cause interference with or failure of surrounding electronic equipment. Conversely, the electronic device may be damaged or disrupted by incoming EMI and/or RFI emissions produced by the nearby electronic equipment. As such, electronic devices are typically subject to government regulations related to outgoing levels of, and susceptibility, to EMI and RFI radiation. Therefore, such equipment and systems must be designed to be in compliance with the standards set forth by the government.
A conventional approach to reducing both emissions of, and susceptibility to, EMI and RFI is to enclose the electronic device in a continuous, electrically-conductive enclosure which is connected to earth ground potential. Such an enclosure, known in the art as a xe2x80x9cFaraday cagexe2x80x9d, intercepts EMI and RFI radiation, whether emitted or received, and converts it to electrical currents at the surface of the enclosure. The currents then pass to earth ground without harmful effects according to electromagnetic effects which are well-known in the art. Such enclosures need not be truly continuous in their construction. Some apertures in the enclosure may be tolerated depending upon the size of the aperture and its relation to the frequency or frequencies of the radiation being attenuated. In general, the higher the frequencies, the smaller the size of the aperture which may be tolerated. In this manner, necessary openings, such as for air cooling, may be designed into the enclosure without unacceptably comprising the attenuation characteristics of the structure.
The construction of such an enclosure, including the methods by which parts of the enclosure are formed and attached together, has an impact on the attenuation which can be performed. Ideally, all parts of the enclosure are continuously and invariably connected together by wide and homogenous conductive paths. Points of disruption in the continuous enclosure may form electrical defects having properties of capacitance and/or inductance. The effect of such defects may concentrate the flow of the aforementioned surface currents to an intensity sufficient to cause them to re-radiate from the surface. As such, an electrical defect in the enclosure may act as an antenna which concentrates and re-radiates the EMI or RFI radiation that had previously been attenuated. Thus, these defects may render the entire enclosure without effect.
A very common site for defects to occur is at points where portions of the enclosure are fastened or attached by separable connections. Connecting methods, such as the use of mechanical fasteners (e.g., screws and rivets) and intermittently-welded joints, offer opportunities for such electrical defects to occur, either in construction or in the course of time. These defects are especially prone to occur at joints or connections which are regularly separated, such as in the use of the device for service attention. Poorly designed separable connections between parts of the enclosure may degrade over time and use to the point where the entire enclosure becomes ineffective.
Even well-designed connections may be degraded over the course of time due to surface corrosion. Specifically, oxide layers may form on the surfaces of mating parts. Metallic oxides act as dielectric materials and can cause a capacitor to form at the connection. As previously described, a re-radiating defect may result. For this reason, costly non-corrosive metals such as gold, silver and nickel are often used at the joints and separable connections of enclosures.
A more significant problem in the construction of such enclosures involves the use of large openings for functional or regular service access. Such openings typically require some form of closure, such as a door or panel, to prevent the passage of EMI and RFI into or out of the enclosure. This closure must be designed and constructed so as to form an electrically-continuous part of the enclosure when secured in place. To this end, conductive means such as electrically-conductive compressible mesh, rubber gaskets, or flexible conductive xe2x80x9cfingersxe2x80x9d are conventionally used at the joints of the closure. Fastening means are used to install the closures in order to ensure continuous electrical contact at all points along its edges. The conductive means typically require considerable compressive forces between mating parts to obtain the desired conductive action, which only serves to increase the size and number of fastening means required to ensure a reliable connection. As such, a large removable closure, conductively sealed and attached by conventional means, may require the use of a fastener every one to two inches along its entire periphery. In the case of a suitable access closure for allowing upper-body access, this may result in a closure which requires a hundred or more fasteners to install, at great cost and labor expense.
The problem becomes even more acute in the case of highly-automated systems, where it is required that sealing closures be automatically opened and closed. This would be the case, for example, with a large automated test machine, which requires complete EMI/RFI shielding while it is in operation, but which also requires large removable closures in the EMI shield which can be opened to access the machine inside as for example, to load or unload the devices to be tested. In such cases, where the functions of the machine are entirely automated, the actions of opening, closing and electrically sealing the closures must also be powered and automated.
To address these and other difficulties, the present invention is directed toward an electrically-continuous EMI/RFI rotary seal system for closures in EMI-impermeable enclosures. Specifically, in an enclosure having an interface between a fixed portion and a movable portion, an EMI seal mechanism comprises a support mechanism coupled to one of the fixed portion and the movable portion. The support mechanism is positionable from an electrically non-conductive position to an electrically conductive position, wherein the electrically conductive position provides a substantially continuous electrical connection through the joint between the enclosure parts.
In addition, rotary action of the support mechanism ensures that with each cycle of the device, a frictional force is generated between conductive elements and their respective contact surfaces. This force is tangential to the ends of the various fingers, thus generating a wiping or cleaning action on both the fingers and the contact surfaces. This action eliminates the formation of an oxide film and ensures continued electrical contact without degradation.